Generally, a solid-oxide fuel cell system is a system that generates electric power in such a manner that: a hydrogen-containing gas and air are supplied to a fuel cell that is a main body of an electric power generating portion; and chemical energy generated by an electrochemical reaction between hydrogen and oxygen in the air is taken out as electric energy. During a steady operation of the solid-oxide fuel cell system, a solid-oxide fuel cell operates at a high temperature of 500 to 900° C.
The solid-oxide fuel cell system includes a hydrogen generator configured to generate the hydrogen-containing gas (reformed gas). As a raw material (electric power generation raw material) for generating the hydrogen-containing gas, the hydrogen generator uses a fossil material, such as a city gas or LPG which contains a natural gas as a major component. The hydrogen generator includes a reformer, and the reformer generates the hydrogen-containing gas by a reaction (reforming reaction) between the electric power generation raw material and steam at a high temperature around 600° C. by using, for example, a Ru catalyst or a Ni catalyst. During the steady operation of the solid-oxide fuel cell system, the reformer is maintained at a high temperature of 400 to 700° C. and is continuously supplied with the electric power generation raw material and water or the like to generate the hydrogen-containing gas by the reforming reaction using the catalyst.
The solid-oxide fuel cell system includes an evaporator that generates steam from water supplied from outside, the steam being necessary in the reforming reaction in the reformer. During the steady operation of the solid-oxide fuel cell system, the evaporator is maintained at a high temperature of 100 to 300° C.
In an operation stop process executed when stopping the operation of the solid-oxide fuel cell system, the fuel cell, the reformer, the evaporator, and the like which are operating at high temperatures need to be cooled to predetermined temperatures, and the hydrogen-containing gas remaining in the reformer, the fuel cell, and channels through which the hydrogen-containing gas flows needs to be purged. The reasons for this are as below. The remaining hydrogen-containing gas contains steam. Therefore, when the temperature of the hydrogen-containing gas becomes a dew point or lower in a cooling process, the steam condenses into water. At this time, air intrudes from outside by pressure decrease. Therefore, an anode material is oxidized by the air. On this account, by repeating start-up and stop, the oxidation and reduction of the anode material are repeated. This becomes a cause of significantly deteriorating durability of the anode material. Further, the condensed water becomes a cause of significantly deteriorating durability of the catalyst filled in the reformer or a desulfurizer in addition to the anode. It should be noted that a process which is executed when stopping the operation of the solid-oxide fuel cell system and includes a plurality of processing steps such as the purge is referred to as the operation stop process. The operation stop process is a process from when an electric power generation stop instruction is received until when the supply of the hydrogen-containing gas and the supply of an oxidizing gas are stopped. The supply stop of the hydrogen-containing gas and the supply stop of the oxidizing gas may be performed when, for example, the temperature of a stack of the solid-oxide fuel cell or the temperature of the reformer reaches a predetermined temperature (100° C., for example).
Conventionally known is a solid-oxide fuel cell system in which in the operation stop process, the purge of the hydrogen-containing gas is performed by forming a reduction atmosphere using an inactive gas such as nitrogen. However, when the purge is performed using the inactive gas, channels dedicated for the purge need to be provided, and this causes a problem in which the solid-oxide fuel cell system increases in size. Therefore, proposed is a fuel cell system in which the purge of the hydrogen-containing gas is performed by using a raw material gas (PTL 1, for example).